A Distributed Antenna System (DAS) generally refers to a radio access architecture consisting of a large number of antennas (or radio heads) distributed widely across a large area and connected to a centralized processing center. Each group of antennas covers an area commonly referred to as a cell. The total area covered by the multiple antennas connecting to the same central unit is referred to as a DAS cell cluster, to be distinguished from the cell in a conventional system. Unlike a conventional cellular system in which signals transmitted and received by each antenna are processed individually at the base station attached thereto, a DAS has complete control of all the widely distributed antennas and can therefore coordinate their transmission and reception to minimize interference and achieve high capacity. More recent advancements in distributed antenna systems have given rise to the CoMP system (Coordinated Multipoint Transmission and Reception) used in 3GPP (3rd Generation Partnership Project). A CoMP cell cluster also includes several cells each of which has one or more antennas for providing radio coverage.
DAS and CoMP systems must have sufficient knowledge of the channel responses between the terminals and the distributed antennas to effectively suppress interference. Such knowledge is typically estimated through the uplink transmission of known signals commonly referred to as sounding reference signals, reference signals or pilot signals, hereinafter referred to generally as reference signals. The accuracy of the channel estimates depends heavily on the selection and deployment of the uplink reference signals. Uplink reference signal allocation and assignment becomes even more complicated when there are multiple neighboring DAS or CoMP cell clusters with non-coordinating processing centers, i.e., processing centers which do not coordinate transmissions amongst cell clusters.
Accurate uplink channel estimates are essential to the coherent demodulation of uplink data transmitted by terminals served within a cell cluster. Accurate uplink channel estimates are also critical for downlink transmission in the case of Time Division Duplex (TDD) systems which can derive the downlink channel responses from the uplink channel estimates.
In a conventional system such as Long Term Evolution (LTE), each base station is allocated a set of uplink reference signals which is orthogonal to those allocated to a neighboring cell. The base station then assigns uplink reference signals selected from this set to the terminals it is serving. The base station uses reference signals received from the terminals to perform channel estimation and perform coherent demodulation. Neighboring cells are allocated mutually exclusive sets whereas the same set of signals can be reused by other cells that are separated so that no or only limited interference occurs between the cells. Examples for the orthogonal reference signal set can be different cyclic shifts of a base sequence such as Zadoff-Chu sequence, m-sequence or Costas sequence. Another possibility is that the uplink reference signals are made orthogonal in the time-frequency domain for an Orthogonal Frequency Division Multiplexing (OFDM) system.
In each case, the static reuse of orthogonal reference signal sets may be sufficient for a conventional system in which each base station serves only one terminal at a time. However, for a DAS or CoMP cell cluster, multiple users are served simultaneously by a processing center. The number of orthogonal signals may not be sufficient to adequately serve all terminals if a conventional static reuse method is adopted at the DAS or CoMP cell level. Furthermore, restricting the interior cells of a DAS or CoMP cell cluster from reusing signals allocated to a neighboring cell cluster is unnecessary since the interior cells are shielded by the boundary antennas of the cell cluster. A conventional static reuse approach can be adopted at the antenna level. In other words, the same orthogonal reference signal set can be reused by the antennas that are separated far apart. However, this static reuse within a DAS or CoMP cell cluster fails to fully exploit the ability of the processing center to dynamically optimize pilot signal assignments.